Inhibition of phosphoric acid corrosion



p 22, 1953 J. D. KEMP ET AL ,653,177

INHIBITION OF PHOSPHORIC ACID CORROSION Filed June 4, 1949 AISI TYPE 3|?STEEL INCHES PENETRATION PER I000 HRS. 9

9O 95 I00 I05 PERCENT PHOSPHORIC ACID (H PO INVENTORS Jacob D. KempHenry R Zeh BYZ ATTORNEYS Patented Sept. 22, 1953 UNITED STATES PATENTOFFICE INHIBITION OF PHOSPHO'RIC ACID CORROSION Delaware ApplicationJune 4, 1949, Serial No. 97,287

1 Claim.

This invention relates to a method for inhibiting the corrosive attackof concentrated phosphoric acid upon high chrome-nickel austeniticsteels containing molybdenum, and more particularly, to a method ofinhibiting such attack during hydrocarbon reactions catalyzed byphosphoric acid at concentrations above about 100% calculated asorthophosphoric acid and conducted at temperatures above about 200 F. inreaction vessels constructed from or lined with such steels.

Liquid phosphoric acid is known to be an efilcient catalyst for certainorganic reactions such as the alkylation of aromatics with olefins, theisomerization of olefins, and especially for the polymerization ofnormally gaseous olefins. The corrosive attack of phosphoric acid uponthe various materials available for construction of reaction vesselssuitable for conducting such reactions is so rapid that this catalysthas not heretofore been employed in commercial alkylation,isomerization, or polymerization units.

It is an object of this invention to provide a method for inhibiting theattack of concentrated phosphoric acid on high chrome-nickel austeniticsteels containing molybdenum.

It is a further object of this invention to provide a combination of amaterial of construction, an acid concentration of phosphoric acid, anda corrosion inhibitor which makes possible the commercial polymerizationof normally gaseous olefins catalyzed by liquid phosphoric acid in whichthe process hazards and unfavorable economic effects of corrosion aresubstantially eliminated.

High chrome-nickel austenitic steels containing molybdenum are readilyavailable as materials for the construction of commercial scale reactionvessels. They are inherently acid resistant, and problems of fabricatingof these steels are well understood. They have been successfullyemployed in the construction of a variety of commercial articles andvessels large and small and are eminently suitable for the constructionof reaction vessels and connecting lines for 2 processes employingconcentrated phosphoric acid as a catalyst when the rate of theircorrosion is reduced in the manner hereinafter described.

By the term high chrome-nickel austenitic steels containing molybdenumas used herein, it is intended to include austenitic steels containingat least 16% and up to about 30% of chromium, containing at least 7% andup to about 20% of nickel, containing at least of 1% and up to about 8%and preferably between 1% and 5% of molybdenum and containing not inexcess of of 1% carbon. AISI types 316 and 31'! steel are illustrativeof commercially availabl high chrome-nickel austenitic alloys containingmolybdenum. Type 316 contains 16 to 18% chromium, 10 to 14% nickel, 2 to3% molybdenum, a maximum of of 1% carbon, and the remainder iron. Type31'? metal contains 18 to 20% chromium, 10 to 14% nickel, 3 to 4%molybdenum, a maximum of of 1% carbon, and the remainder iron. These twoparticular alloys and other alloys falling within the broader range ofcomposition set forth above may be made highly resistant to thecorrosive attack of concentrated phosphoric acid by the addition to theacid of minor amounts of dissolved copper.

It has been found that the attack by phosphoric acid on highchrome-nickel steels containing molybdenum is markedly decreased at acidconcentrations above about 100% calculated as orthophosphoric acid. Anumber of test runs were made in which type 317 steel was exposed to theattack of concentrated phosphoric acid at concentrations in the range-115% and the temperatures within the range ZOO-400 F. The concentrationof the phosphoric acid is determined by titrating a weighed sample ofacid with a base of known concentration. From the titration the weightof orthophosphoric acid equivalent to the base consumed is calculatedand the concentration of the sample is then determined by dividing thecalculated weight by the measured weight of the sample and expressingthe result as per cent orthophosphoric acid.

The results of these tests are summarized in the attached figure, whichis a graphical representation of the variation of corrosion rate withacid concentration at a variety of temperatures. From the figure it isseen that as the acid concentration rises above about 95% and especiallywhen it is above 100%, the rate of corrosive attack decreased veryrapidly as the acid concentration is increased. The figure clearly showsthat a. considerable degree of control may be exercised over the rate atwhich high chromenickel austenitic alloys containing molybdenum areattacked by phosphoric acid by making a suitable selection oftemperature and acid concentration. A maximum tolerable corrosion rateof 0.01 inch per thousand hours may be set, based on considerations ofeconomy and safety, and if corrosion is maintained at or below thisrate, a process life of approximately 5 years can be obtained in avessel or pipe having ahigh chromenickel lining inch in thickness. Ifthe acid concentration employed in the process is sufficiently high, asindicated by the attached figure, tolerably low corrosion may be insuredat temperatures up to about 325 F. Where th acid is employed as acatalyst for the polymerization of normally gaseous olefins, this methodof controlling corrosion requires the sacrifice of a considerable degreeof process flexibility, since the high acid concentrations which arerequired to control corrosion are accompanied by a tendency toward theproduction of a heavier polymer product than could be obtained withacids at lower concentration.

7 In addition to the effect of acid concentration on the corrosion rateindicated above, it has been found that the addition of small amounts ofcopper to the acid causes a marked decrease in the rate of corrosion ofhigh chrome-nickel austenitic steels containing molybdenum. When copperis dissolved in phosphoric acid at concentrations above 100%, thecombined effects of the high acid concentration and the presence ofcopper on the corrosion rate extends the range of acid concentrationwhich may be employed at any given temperature While maintaining thecorrosion rate well below 0.01 inch per thousand hours. The use ofcopper with very concentrated phosphoric acid, for example 110% acid,makes it possible to conduct the catalytic polymerization of olefins attemperatures up to about 375 F. without exceeding 0.01 inch of corrosivepenetration per 1000 hours. This combined effect increases theflexibility of a process such. as polymerization of normally gaseousolefins and greatly extends the duration of the apparatus life.

It hasfurther been found that if the high chrome-nickel austeniticsteels containing molybdenum have either a low carbon content below0.03% or contain columbium in an amount equal to about 10 times thecarbon content, but not greater than 1%, or contain titanium in anamount equal to about 4 times the carbon content but not greater than1%, then, not only is ordinary corrosion occasioned by the attack of theacid reduced by the addition of copper, but also intergranular corrosiondoes not occur. This is a significant aspect of the employment of copperto inhibit corrosion of high chromenickel austenitic alloys byphosphoric acid, since it is lrnown that the immersion of stainlesssteels in a boiling solution of sulfuric acid and copper sulfate(Strauss solution) greatly accelerates the rate of intergranularcorrosion. It is believed significant and serious where the materialunder consideration is used in the construction of vessels in whichreactions are carried out at an elevated pressure.

A series of tests was conducted in which ordinary corrosion rates andthe incidence of intergranular corrosion were determined in regular AISItype 316 and 317 steels containing about 0.07% carbon. Half of thespecimens were heat treated at 1100 F. for 24 hours tomake. them highlysusceptible to intergranular corrosion and the other half were given astress-relief treatment at 1300 for 7 hours. All the specimens wereexposed for a total of 840 hours in 104% phosphoric acid at 300 F. Theresults of these tests are summarized in the following Table I.

TABLE I Corrosion resistance of regular type 316 and 317 steels exposedin 104% phosphoric acid at 300 F. for 840 hours Corrosion Rate inHotcnal Heat Treatment solved gconm lnches Copper/l. sion Pm 1,000 HoursType 314 l,l50 F. for 24 hrs-" 0. (1 0, 016 Do .do. .l 0.1 .010 .i0r7hrs0.1 .009 for 24 hrs 0. 3 .007. .for7hrs l 0.3 .007 for 24 hrs-.. 0.0.002 for 7 hrs 0.0 .0015 for 24 hrs 2.6 .003 do 0.1 .006

.f0r7hrS., 0.1 .006 for 24 hrs". 0v 3 .005 F. for 7 hrs 0.3 005 for24l1rs 0.9 .006 l'or7 hrsm. 0.9 .005 for 21 hrs.. 2. 6 .004

Data summarized in the above table show that the reduction in corrosionby the addition of one gram of copper per liter of acid was veryeffective for regular type 317 steel; the rate was 0.002 inch perthousand hours. The lowest rate observed for the regular type 316 steelwas 0.005 inch per,

thousand hours; in this case the copper addition was not as effective asin the-case of the type 317. The type 317 steels appeared to becompletely immune to intergranular corrosion, whereas the type 316specimens were not. It is not believed that the additional molybdenum inthe type 317 steel was responsible for its immunity to intergranularcorrosion, since molybdenum usually has little efiect in this direction.Rather, it is believed that longer exposure of the type 317 steel wouldhave produced incipient intergranular corrosion. The addition of copperclearly reduces the ordinary rate of corrosion of regular type 316 and317 steels, but if their use in pressure apparatus were contemplated,intergranular corrosion might present a hazard.

In a second series of tests, 14 samples of type 316 and type 317 steelshaving either a low carbon content below 0.03 C2, or containing up to 1%of columbium, the columbium content being approximately 10 timcs thecarbon content, were heat treated at 1100" F. for 24 hours to inducesusceptibility to intergranular corrosion, and were then exposed to 104phosphoric acid con taining approximately 1 gram per liter of dis solvedcopper at 300 F. for 840 hours.

The results of this series of tests are summarized in the followingTable II:

TABLE II Efiect of copper on the corrosion resistance of low carbon(0.03 mar.) and columhium (1 max.) stabilized type 316 and type 317steefs exposed in 104% phosphoric acid at 300 F. for 840 hours [The acidcontained 0.9 gram per liter of dissolved copper ll all of the tests.All specimens were heated [or 24 hours at 1150 F. to obtain maximumsensitivity to intergranular corrosion] Corrosion lntcrgran- Rate inMaterial ulor Cor- Inches rosion Tcn. Per

1 .000 Iiou rs Type 316 low'O i 0.002 Do .002 D0 .001 Type 317 001 D0.002 D0 .003 D0 00l Type 316 CI).... .0015

D0. .002 D0 00-1 Do .001 D0. .003 Type 317 Ch .00.; Type 317, 0.04 C0015 1 Microscopic examination indicated that this alloy did not containsufficient columbium for stabilization.

The results summarized in the above table show that all of the alloyscontaining 0.03% carbon or less and also those containing sufilcientcolumbium for stabilization were resistant to in TABLE III Eflect ofcopper on the corrosion resistance of regular and low carbon type 317steels in 104% phosphoric acid at 350 F. for .20 hours Corrosion Gramsof Hate in Material Dissolved Inches Pen.

Copper/l. Per 1,000

Hours Type .il7 0.0 0. 18 Type 317 low C 0.0 .22 Type 317 0.1 .047 Type317 low C 0.1 .053 Type 3l7 0. 3 .031 Type 317 low C. 0.3 .038 Type 3170.9 .000 Type 3l7low C 0. 9 .012

From the foregoing data, it will be seen that the addition of copperreduced the corrosion rate from 0.2 inch to 0.012 inch per thousandhours when about 1 gram of copper per liter of acid was present.Corrosion rates of the low carbon alloy are slightly higher than thoseof the regular alloy, but the diiference is not significant.

From the data obtained in similar experiments, it is concluded that thepresence of dissolved copper in amounts in excess of about 0.05 gram perliter of acid have a significant effect in reduc 6 ing the ordinarycorrosive attack of phosphoric acid upon high chrome-nickel alloyscontaining molybdenum. This reduction in ordinary corrosion isaccompanied by an absence of any tendency toward intergranularcorrosion, where either the carbon content of the alloys is below 0.03%or where stabilizing amounts of columbium or titanium up to about 1% areincluded in the alloy.

The amount of copper which is added to the acid to inhibit corrosion inthis manner is preferably in the range 0.05 gram to 2.5 grams of copperper liter of acid, the usual amount being approximately 1 gram perliter. Larger quantities of copper might be added, but relatively littleeffect is produced on the rate of corrosion by the addition of copper inamounts in excess of about 1 gram per liter. Further, there is anindication that the addition of larger amounts of copper may undesirablyaffect the quality of the polymer product produced when the acidcontaining the dissolved copper is employed to catalyze thepolymerization of normally gaseous olefins.

The copper is added to the acid usually in the form of copper phosphate(C113(PO4)2-3H2O) since the addition of this compound does not involvethe introduction of negative ions other than the phosphate ion into theacid catalyst. Since the amount of the copper compound added is verysmall, other copper compounds than the phosphate may be added to theacid without producing any significant change in the catalytic activityof the acid-copper solution.

The employment of 0.05 to 2.5 grams per liter of dissolved cop-per inphosphoric acid at concentrations of 103.2%, 104.9%, 105.4%, 106.4%, and110.3% calculated as orthophosphoric acid was found to inhibit corrosionof type 316 and type 317 steels during the polymerization of propyleneand of propylene-butene mixtures at temperatures in the range 200 to 375F. and at pressures in excess of 200 p. s. i. g. The employment of thecopper inhibitor in the acid catalyst in these amounts was not found tohave any detectable efiect on conversion rates of the mono-olefins noron the quality or" the polymer produced.

The following example illustrates the efiectiveness of copper to inhibitthe corrosion of type 31'? steel during the polymerization of propylene.A gas consisting essentially of propylene and propan and containing44.2% by volume of propylene was intimately contacted with phosphoricacid in a continuous run of 33 hours duration. The gas introduced intothe acid in liquid phase and the acid and hydrocarbon were vigorouslystirred together. A mixture of acid and hydrocarbon was continuouslywithdrawn and settled. The hydrocarbon was removed as a product and theacid was returned to the reactor. The acid had an initial concentrationof 103.2% and contained 2.1 grams of dissolved copper er liter of acid.The reaction was conducted at 300 F. and at a pressure of 360 p. s. i.The feed was introduced at a rate of 1.7 volumes of liquid propylene pervolume of acid per hour and conversion was 85.2%. The product consistedpredominantly of Cs-C1s polymers and had an end point of 510 F. with 99%overhead. The corrosion rate of type 317 metal during the run was 0.0018inch per 1000 hours.

Copper was similarly efiective in reducing corrosion of highchrome-nickel austenitic steels containing molybdenum during thepolymerization of butenes and mixed propylene-butene feeds by contactwith phosphoric acid at 100% and higher concentrations.

We claim: Number Name Date In an organic conversion process catalyzed by2,005,861 Ipatiefl June 25,1935 phosphoric acid at concentrations inexcess of 2,135,793 Brooke Nov. 8, 1938 100% calculated asorthophosphoric acid con- 2,462,638 Hetherington Feb. 22, 1949 ducted attemperatures of at least 200 F. and 5 2,482,104 Dolian Sept. 20, I949super-atmospheric pressures in a reaction vessel presenting metalsurfaces in contact with said FOREIGN PATENTS phosphoric acid of anaustenitic steel containing Number Country Date 16-30% chromium, 720%nickel, at least 450 5 Great Britain July 20, 1936 molybdenum and not inexcess of 0.03% carbon, 10 517,799 Great Britain Feb. 8, 1940 the methodof inhibiting corrosive attack of said steel by said acid withoutinducing intergranular OTHER REFERENCES corrosion which comprisesmaintaining in solu- Rohrman: Corrosion of Metals by Phosphoric tion insaid acid a minor amount of copper be- Acid, Chem. and Met. Eng, vol.42, No. '7, pages tween about 0.05 and 2.5 grams per liter of acid. 15368-9, (July 1935)- JACOB D. KEMP. Harford et al.: Corrosion of Metals,Ind. and HENRY P. ZEH. Eng. Chem, vol. 31, No. 9, pages 1123-28, (Sept.

1939). References Cited in the file of this patent Kinzel et a1: Alloysof Iron and Chromium,

UNITED STATES PATENTS 20 V01. 11, pages 413-437, (McGraw-Hill 1940)Number Name Date 1,809,041 Jenkins et a1 June 9, 1931

